However, I recently changed to a new ISP (can't go back). Now everything is fine except gaming. Every little while (it seems random) my game (ET: Quake Wars) lags out for a minute or so (sometimes longer) then everything returns to normal. The gaming computer is connected using a wired ethernet connection.

I tested gaming without the router and there is no lag, so I'm sure its a router issue (well a router issue related to my ISP).

While I'm lagging out (my ping goes dances from 300-1000+) I can still have a VOIP conversation at the same time (with no dropouts). So it seems as if only the game's packets are being dropped/lost.

I tried upgrading from Thibor 15c (Hyper 2.1b1) to Tomato 1.13 and there is no improvement. I also played around with the Qos settings but couldn't fix the problem.

Well I solved this myself. After troubleshooting every single possible problem, I concluded that it is a Router problem.

Whether my router is faulty (everything else but gaming works perfectly and it worked fine on my old ISP) or whether this router just can't handle the game (doubtful since others don't seem to have any problems)... I decided to buy myself a new router. Since Linksys has generally pissed me off on my last two purchases from them, I bought a D-Link DGL-4300. The setup is crap but now that its working... no more problems with gaming and it has all the features I could ever need.